Hybrid reflective-emissive display for use as a signal light

ABSTRACT

A hybrid reflective-emissive display including an electrophoretic medium. The electrophoretic medium includes two types of oppositely-charged, and differently-colored, particles in a fluid. The electrophoretic medium is disposed between a front light-transmissive electrode and a rear electrode including a plurality of apertures, such as a hexagonal grid. The electrophoretic layer is optionally illuminated by a light source disposed on the opposite side of the rear electrode from the electrophoretic medium. A viewer can see either the first or second type of particle at the viewing surface, or one of the particles can be illuminated by the light source, thus emitting light the color of one of the particles.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/218,564, filed Dec. 13, 2018, which is a continuation of co-pendingapplication Ser. No. 13/867,633filed on Apr. 22, 2013, now U.S. Pat. No.10,190,743, which claimed the benefit of provisional Applications Ser.Nos. 61/636,070, filed Apr. 20, 2012, and 61/654,405, filed Jun. 1,2012. However, the applicant hereby declares, in accordance with 37 CFR1.78(a)(6), that the text of this application also contains an inventionthat has an effective filing date on or after Mar. 16, 2013.

The entire contents of the aforementioned copending applications, and ofall patents, copending applications, and published applicationsmentioned below are herein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to illumination systems for reflective displays.More specifically, this invention relates to a display and means forprojecting patterned light onto at least a part of the display. Thisinvention also relates to a method for directing spatially andspectrally modulated illumination on to a reflective display to improvecontrast and colorfulness under ambient lighting conditions. Certainembodiments of the present invention relate to a display, intendedprimarily for outdoor use but which may also find some applicationindoors (in the sense of use within buildings, tents and other similarstructures); these embodiments of the display of the present inventionmake use of a reflective bistable electro-optic display in conjunctionwith a light source arranged to illuminate the reflective electro-opticdisplay.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a drive scheme which only drives pixels to their two extremeoptical states with no intervening gray states.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in U.S. Pat. No. 7,420,549 that such electro-wettingdisplays can be made bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thethese patents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728 and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;    -   (d) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        D485,294; 6,124,851; 6,130,773; 6,177,921; 6,232,950; 6,252,564;        6,312,304; 6,312,971; 6,376,828; 6,392,786; 6,413,790;        6,422,687; 6,445,374; 6,480,182; 6,498,114; 6,506,438;        6,518,949; 6,521,489; 6,535,197; 6,545,291; 6,639,578;        6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,724,519;        6,750,473; 6,816,147; 6,819,471; 6,825,068; 6,831,769;        6,842,167; 6,842,279; 6,842,657; 6,865,010; 6,967,640;        6,980,196; 7,012,735; 7,030,412; 7,075,703; 7,106,296;        7,110,163; 7,116,318; 7,148,128; 7,167,155; 7,173,752;        7,176,880; 7,190,008; 7,206,119; 7,223,672; 7,230,751;        7,256,766; 7,259,744; 7,280,094; 7,327,511; 7,349,148;        7,352,353; 7,365,394; 7,365,733; 7,382,363; 7,388,572;        7,442,587; 7,492,497; 7,535,624; 7,551,346; 7,554,712;        7,583,427; 7,598,173; 7,605,799; 7,636,191; 7,649,674;        7,667,886; 7,672,040; 7,688,497; 7,733, 335; 7,785,988;        7,843,626; 7,859,637; 7,893,435; 7,898,717; 7,957,053;        7,986,450; 8,009,344; 8,027,081; 8,049,947; 8,077,141;        8,089,453; 8,208,193; and 8,373,211; and U.S. Patent        Applications Publication Nos. 2002/0060321; 2004/0105036;        2005/0122306; 2005/0122563; 2007/0052757; 2007/0097489;        2007/0109219; 2007/0211002; 2009/0122389; 2009/0315044;        2010/0265239; 2011/0026101; 2011/0140744; 2011/0187683;        2011/0187689; 2011/0286082; 2011/0286086; 2011/0292319;        2011/0292493; 2011/0292494; 2011/0297309; 2011/0310459; and        2012/0182599; and International Application Publication No. WO        00/38000; European Patents Nos. 1,099,207 B1 and 1,145,072 B1;    -   (e) Color formation and color adjustment; see for example U.S.        Pat. Nos. 6,017,584; 6,664,944; 6,864,875; 7,075,502; 7,167,155;        7,667,684; 7,791,789; 7,956,841; 8,040,594; 8,054,526;        8,098,418; 8,213,076; and 8,363,299; and U.S. Patent        Applications Publication Nos. 2004/0263947; 2007/0109219;        2007/0223079; 2008/0023332; 2008/0043318; 2008/0048970;        2009/0004442; 2009/0225398; 2010/0103502; 2010/0156780;        2011/0164307; 2011/0195629; 2011/0310461; 2012/0008188;        2012/0019898; 2012/0075687; 2012/0081779; 2012/0134009;        2012/0182597; 2012/0212462; 2012/0157269; and 2012/0326957;    -   (f) Methods for driving displays; see for example U.S. Pat. Nos.        7,012,600 and 7,453,445;    -   (g) Applications of displays; see for example U.S. Pat. Nos.        6,118,426; 6,473,072; 6,704,133; 6,710,540; 6,738,050;        6,825,829; 7,030,854; 7,119,759; 7,312,784; 8,009,348;        7,705,824; and 8,064,962; and U.S. Patent Applications        Publication Nos. 2002/0090980; 2004/0119681; 2007/0285385; and        2010/0201651 ; and International Application Publication No. WO        00/36560; and    -   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; 6,950,220; 7,420,549 and 8,319,759; and U.S. Patent        Application Publication No. 2012/0293858.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

Other types of electro-optic media may also be used in the displays ofthe present invention.

Regardless of the exact technology used to display data thereon,electro-optic displays may be grouped functionally into two broadcategories, namely emissive displays, in which the light is emitted fromor transmitted through the active layer, and reflective displays, inwhich light is reflected from the active layer. An emissive displayconveys information by changes in its luminance and may be viewed in theabsence of ambient light, whereas a reflective display conveysinformation by changes in its reflectance and cannot be viewed withoutambient light. Emissive displays may incorporate materials that areintrinsically electroluminescent (for example, organic light-emittingdiodes, OLEDs) or may be constructed by combining a transmissive orreflective light modulator with a light source; for example, liquidcrystal displays (LCDs) commonly combine a non-emissive light valvelayer with a backlight. Digital projectors may be regarded as emissivedisplays comprising a high-intensity light source and a light modulatorplus appropriate lenses to deliver an image to a distant, reflectivesurface. All emissive displays have the disadvantage that their contrastand colorfulness both depend upon the intensity of ambient light. Invery bright environments, such as sunlight, the emitted light may beoverwhelmed and the displayed information difficult to see. Reflectivedisplays have the advantage that their contrast and colorfulness are notaffected by the level of ambient light, indeed their contrast andcolorfulness even improve in very bright light. However, reflectivedisplays are obviously difficult to see in dim light.

A further difficulty arises with reflective displays intended to rendercolored images. As described, for example, in U.S. Pat. No. 8,054,526, acolor filter array may be positioned so that an appropriateblack-and-white image is viewed through the color filter array. Althougha color image is thus provided to the viewer, the color filtersnecessarily reduce the amount of light reflected from the display in thewhite state, and the necessary sharing of the available display surfacearea between the different color primaries limits colorfulness and colorgamut.

Numerous attempts have been made to construct hybrid emissive/reflectivedisplays visible in any ambient light. For example, U.S. Pat. No.7,170,506 describes a hybrid emissive/reflective display in which theintensity of the emitted light is adjustable. As previously noted, thelegibility of information on an emissive display suffers under verybright ambient light, so that the intrinsic luminance of the displaymust be increased to counter the resultant loss of contrast andcolorfulness. Conversely, the legibility of information on a reflectivedisplay suffers under very low lighting conditions so that theillumination of the display must be increased accordingly to counter theloss of contrast and colorfulness.

Technical solutions have been developed to increase the contrast ratioof emissive displays, so such displays can display high dynamic range(HDR) still and moving images. Some of these so-called HDR displayscombine an LCD color panel with a bright backlight, and modulate thisbacklight with image information, usually in the form of a low-passfiltered luminance channel (this approach is sometimes termed “localdimming”). In more general terms, HDR display can be accomplished bycombining two low dynamic range (or low contrast ratio) devices, and bygenerating from the HDR image two low dynamic range images for each ofthe display devices. Examples of modulated backlights include thedigital projectors or LED arrays described in U.S. Patent ApplicationsPublication Nos. 2008/0137990 and 2008/0137976. However, no equivalentsolution is currently known to increase the contrast and colorfulness ofreflective displays.

It is known that spatially congruent combinations of projected andreflective images can have contrast ratios exceeding that of either theseparate projected and reflective images. A solution that aims atincreasing the contrast of projected images is described in U.S. Pat.No. 6,853,486 (the '486 patent). This patent describes an activeprojection screen that is in registration with the projected image andis used to increase the contrast of the projected image under brightambient light that would otherwise diminish the contrast of the sameimage projected on to a uniformly reflective screen. The main drawbackto this system is the need to maintain exact registration between theactive projection screen and the projector. Without rigid mechanicalcoupling of projector and screen the required exact alignment andregistration are very difficult to achieve and maintain, and thesolution described in the '486 patent, namely electronic couplingbetween projector and screen via ‘reflectance processor’ and ‘displaycontroller’, is expensive.

The system described in the '486 patent is a large-size emissive(projection) display, and the patent does not disclose any device inwhich color projection means image and means for rendering a reflectiveimage are incorporated into the same device, such that both images canbe rendered in registration with each other. In one aspect, the presentinvention seeks to increase the contrast and colorfulness of reflectivedisplays, especially hand-held devices, under a wide range of ambientlighting conditions, by combining projected and reflective images.

One specific application where bistable electro-optic displays may beuseful is outdoor signs, especially traffic control devices.Historically, traffic light and other traffic control signals have beenrelied upon incandescent bulbs to generate light; more recently, lightemitting diodes (LED's) have begun to be used for this purpose. Bothincandescent bulbs and LED's (and indeed, all other emissive lightsources) require a continuous source of power, typically mainsalternating current power, so that any disruption of the power supplydue to equipment failure, weather conditions or traffic accidents willresult in failure of the traffic lights, traffic hazards and majordisruption of traffic flow. Conventional traffic signals have otherdisadvantages, including:

-   -   (a) false signals can occur as a result of sun light and solar        glare; conventional traffic lights must overcome ambient light        conditions, including specular reflections from various surfaces        of the sign, which can make it difficult to discriminate between        off and on states of a particular sign, to be noticeable and        effective; even the common use of light baffles and high powered        LED or incandescent lamps in the range of 25-100 W do not        entirely overcome such problems;    -   (b) traffic lights are located outdoors and hence are subject to        harsh mechanical and environmental conditions; they must        withstand mechanical damage and remain operational despite        vandalism, mechanical shock and impact, extreme temperatures,        and exposure to ultraviolet radiation;    -   (c) total cost of ownership, particularly operating costs, is        very important factor in traffic light usage; in New York city        alone there are 11,871 traffic lights, and substantial effort is        devoted to reducing power usage, including converting        incandescent signals to LED's;    -   (d) strict weight restrictions exist for streetlights to prevent        overloading of signage support structures, so industrial design        and weight allocations must be carefully managed for signage;        and    -   (e) the need (in some cases) for increasing traffic signal size        may compromise performance in terms of power consumption,        weight, and cost.

Accordingly, the present invention seeks to provide a form ofinformation display (which may have the form of a traffic light or othertraffic control device) which overcomes the aforementioned problems ofprior art devices.

Similar problems are encountered when the “traffic control device” is anindicator on an automobile or other vehicle. Even relatively highpowered bulbs do not guarantee sufficient visibility. For example athighway speeds the driver of one automobile may have to react to achange in the brightness of the brake light of a preceding in a fractionof a second if a crash is to be avoided. Most brake lights on cars havea parabolic reflective enclosure to concentrate the light coming fromthe bulb. Although such a parabolic reflector does help to concentratethe light from the brake light into a narrow beam, it also concentratesany light incident on the reflective enclosure (for example, fromsunlight or light from the headlights of a following vehicle) backthrough the colored plastic cover, thus creating background reflectionthat tends to obscure the state of the brake light. In the worst case,when the sun faces the back of the car, the specular reflection of theparabolic reflective enclosure, the reflection of the glossy paint onthe car and the reflection from the rear windscreen combine tosignificantly lower the visibility of the brake light.

In another aspect this invention provides a hybrid emissive-reflectivedisplay which can improve the visibility of vehicle-mounted signs inhigh ambient light conditions, thus providing an added margin of safetyand offering the possibility of lowering power consumption.

SUMMARY OF INVENTION

Accordingly, in one aspect this invention provides a display devicecomprising a reflective display arranged to render a first imageviewable through a viewing surface and a projection means arranged torender a second image viewable in reflection on the viewing surface, thereflective display and the projection means being mounted on a commonframe. This display device may hereinafter be called the “projectiondisplay” of the present invention.

invention also provides a display device comprising a digital projectorand a reflective surface mounted on a common frame, the digitalprojector comprising a light source, a projection lens and at least oneadditional optical element adapted to cause a projected image to beformed on the reflective surface, wherein the light passing from thelight source through the projection lens is folded by more than 180degrees in a plane containing the principal axis of the projection lensand a plane of symmetry of the at least one additional optical elementbefore being reflected from the reflective surface.

This invention also provides a reflective display arranged to render afirst image viewable through a viewing surface and a projection meanscomprising a light modulator arranged to render a second image viewablein reflection on the viewing surface, the first and second images beingsuperimposed and having approximately the same width and height, thefirst image having w1 pixels in the width dimension and h1 pixels in theheight dimension, where

h1>w1

the second image having w2 pixels in the width dimension and h2 pixelsin the height dimension, where

h2<w2.

This invention also provides a display device comprising a reflectivedisplay arranged to render a first image viewable through a viewingsurface and a projection means arranged to render a second imageviewable in reflection on the viewing surface, wherein the frame rate ofthe reflective display differs from that of the projection means by atleast 10 percent.

The foregoing displays devices may hereinafter collectively be referredto as the “projection display” of the present invention.

This invention also provides a method for rendering an image using areflective display arranged to render a first image viewable through aviewing surface and a projection means arranged to render a second imageviewable in reflection on the viewing surface, the method comprising:

-   -   (a) separating the image information into at least two        components, a first component comprising at least luminance        information and a second component comprising chrominance        information; and    -   (b) using the first component to direct the first image and the        second component to direct the second image.

In another aspect, this invention provides an information displaycomprising a bistable reflective display having a display surface and atleast one light emitter arranged to direct light on to the displaysurface of the reflective display. In one form of the invention, thereare at least two separate reflective displays having display surfacesarranged to display differing colors, and independently controllablelight emitters arranged to direct light on to the two display surfaces.This form of the invention may have the form of a traffic light, withthree separate reflective displays with display surfaces arranged todisplay red, amber and green respectively and three light emittersarranged to direct light on to the three display surfaces.Alternatively, this form of the invention might, for example, have theform of a crosswalk sign with two separate display surfaces, one forminga red “DON'T WALK” sign and the other a white “WALK” sign, and two lightemitters arranged to direct light on to the two display surfaces.

It will be appreciated that, in any information display of the presentinvention in which the display surface is to display color rather thansimply black or white, the necessary chrominance may be supplied eitherby the surface itself or by the light emitters. Thus, in a displaydesigned to simulate a conventional traffic light, one could use threedifferent colored electro-optic display areas (using either inherentlycolored media or monochrome media behind color filters) illuminated whennecessary by white light emitters, or three monochrome electro-opticmedia areas illuminated by three different colored light emitters.However, in general the latter is preferred since it provides a highercontrast ratio. The “lit” or colored state of such a monochromeelectro-optic medium/colored light emitter display requires that theelectro-optic medium be set to its reflective (white) state and thelight emitters be turned on so that colored light from the emitters isreflected from the electro-optic medium. In the “unlit” or dark state ofsuch a display, the electro-optic medium is set to its dark,non-reflective state and the light emitters are turned off, thusproducing a very dark display surface and a high contrast ratio betweenthe two states of the display. Although this type of display doesrequire constant use of the light emitters, its power demands can bemade relatively modest; for example, the number of LED's required aslight emitters can be reduced as compared with a conventional LEDtraffic light, since each LED can illuminate a substantial area of thedisplay surface, in contrast to the conventional LED traffic light inwhich the entire display surface must be covered with LED's. The LED'sin such a display of the invention will typically be arranged in a“light ring” around the periphery of a circular display surface, andproviding LED's in such a light ring is likely to be technically simplerthan providing a larger number of LED's in a compact array on thedisplay surface of a conventional LED traffic light. Furthermore,because the light from the LED's spreads relatively uniformly over thereflective display surface, the appearance of the traffic light tends tobe more attractive than that of a conventional LED traffic light, whichsuffers from pixelation due to the visibly separate LED's on its displaysurface.

In another aspect this invention provides an electrophoretic displaycomprising:

-   -   at least one front electrode through which an observer can view        the display;    -   a layer of an electrophoretic medium, the electrophoretic medium        comprising a fluid and two types of electrically charged        particles disposed in the fluid and capable of moving through        the fluid on application of an electric field to the fluid, one        of the two types of particles being dark in color and the other        being reflective and of a color different from that of the dark        particles;    -   at least one rear electrode disposed on the opposed side of the        layer of electrophoretic medium from the front electrode(s), the        rear electrode(s) having a plurality of apertures extending        therethrough; and    -   a light source disposed on the opposed side of the rear        electrodes from the layer of electrophoretic medium and arranged        to direct light through the layer of electrophoretic medium,    -   the display having a first optical state in which the dark        particles lie adjacent the front electrode(s), so that the        observer sees a dark color, a second optical state in which the        reflective particles lie adjacent the front electrode(s), so        that the observer sees the color of the reflective particles,        and a third optical state is which the dark particles lie        adjacent the rear electrode(s), the reflective particles the        light source generates light, and the color of the reflective        particles is visible to an observer.

This invention extends to a vehicle carrying an electrophoretic displayof the invention.

It will be appreciated that in the electrophoretic display of thepresent invention, the so-called “reflective particles” should not becompletely reflective since in the third optical state completelyreflective particles will not allow any light to pass through theelectrophoretic medium. The reflective particles must be sufficientlyreflective to reflect a high proportion of the light incident upon thefront electrode(s) when the display is in its second optical state butstill sufficiently transmissive to allow light to pass through thereflective particles when the display is in its third optical state. Asmay be seen from the detailed Examples below, there is no difficulty infinding commercial pigments which meet these requirements.

The displays of the present invention may make use of any of types ofbistable electro-optic media previously described. Thus, theelectro-optic medium may comprise a rotating bichromal member,electrochromic or electro-wetting material. Alternatively, theelectro-optic medium may comprise an electrophoretic material comprisinga plurality of electrically charged particles disposed in a fluid andcapable of moving through the fluid under the influence of an electricfield. The electrically charged particles and the fluid may be confinedwithin a plurality of capsules or microcells. Alternatively, theelectrically charged particles and the fluid may be present as aplurality of discrete droplets surrounded by a continuous phasecomprising a polymeric material. The fluid may be liquid or gaseous.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1C of the accompanying drawings are schematic diagramsillustrating artifacts associated with grazing angle projection in aprojection display of the present invention.

FIGS. 2A-2C are schematic diagrams showing an arrangement of projectedand reflective images in a projection display of the present invention.

FIG. 3 is a side elevation of a projection engine used in a projectiondisplay of the present invention.

FIG. 4 is a block diagram showing a method of controlling a projectiondisplay of the present invention.

FIG. 5 is a side elevation of part of an information display of thepresent invention in the form of a traffic light, with part of itshousing broken away to show the light sources.

FIG. 6 is a front elevation of the same part of the information displayas shown in FIG. 5.

FIG. 7 is a three quarter view, from in front and to one side, of thesight source assembly of the information display shown in FIGS. 5 and 6.

FIG. 8 is a schematic cross-section through an electrophoretic displayof the present invention having the form of brake light for anautomobile.

FIG. 9 is a front elevation of the rear electrode of the electrophoreticdisplay shown in FIG. 8.

FIG. 10 is a graph of L*a*b* values against time achieved by theelectrophoretic display shown in FIGS. 8 and 9 in certain experimentsdescribed below.

DETAILED DESCRIPTION

As indicated above, in one aspect the present invention provides aprojection display in which means for projecting a color image and meansfor rendering a reflective image are incorporated into a single unitsuch that both images can be superimposed in registration with oneanother, so that a composite image of improved color quality, visible ina wider range of ambient illumination conditions, can be obtained,compared with that which could be rendered by either the reflectivedisplay or the projector alone. In low light, the projected image iseasily visible, its contrast being enhanced by the reflective image onto which it is superimposed. In bright light the projected image willfade but the reflective image will be well lit and seen to goodadvantage. In order to conserve power, it is desirable that theintensity of the projected image be adjusted depending on the ambientillumination (whose intensity can be measured by means well known in theart, such as photodiodes, etc.). In very bright light the projector maybe turned off completely.

The reflective display used in the projection display of the presentinvention can be any of the types previously described, including butnot limited to electrophoretic, electrowetting, electrochromic, rotatingbichromal, and reflective liquid crystal; electrophoretic displays mayfor example be of the magnetophoretic and/or frustrated total internalreflection subtypes. Other types of reflective displays known in theart, for example electronic liquid powder, micromechanical(interferometric), photonic crystal (structural color),electrohydrodynamic, and light valve/reflector, may also be employed.

In one preferred form of the invention, the reflective display comprisesan electrophoretic material comprising a plurality of electricallycharged particles disposed in a fluid and capable of moving through thefluid under the influence of an electric field. The electrically chargedparticles and the fluid may be confined within a plurality of capsulesor microcells. Alternatively, the electrically charged particles and thefluid may be present as a plurality of discrete droplets surrounded by acontinuous phase comprising a polymeric material. The fluid may beliquid or gaseous.

Projection means (“engines”) for use in the projections displays of thepresent invention may use various technologies including light sourcessuch as color light-emitting diodes (LED) and solid state color lasersources, in conjunction with light modulators such as microdisplays madeusing techniques including liquid crystal on silicon (LCoS), deformablemirror displays (DMD) or scanning mirrors (a type ofmicro-electromechanical system, MEMS). Combinations of light source,light modulator and associated optics, such as beam splitters andprojection lenses, are well known in the art, and can be packaged intosuch small dimensions that they are currently referred to aspico-projectors. In preferred forms of the present invention apico-projector is embedded into a mobile reflective display device suchas an electronic document reader (E-reader) or electronic book (E-book)to form a hybrid display.

The technical challenges in combining a pico-projector with a mobilereflective display are very difficult. As a mobile device, the hybriddisplay has to be compact. The embedded pico-projection system must notsubstantially increase the size and weight of the E-reader. Theprojector must not obstruct the view of the screen and the range ofviewing angles of a user reading the display. To provide compactness andunobstructed viewing, angled projection is required. Such projection isused in “short-throw” and “ultra-short-throw” projectors and iswell-known in the art; see for example U.S. Pat. No. 7,239,360. Someartifacts associated with short-throw projection are described in moredetail below.

FIG. 1A shows a projector 10 arranged to render an image on to areflective screen 12 using angled projection, a term used to mean thatthe central chief ray has an angle of incidence greater than zero (andpreferably, in the present invention, at least about 60°) to the normalto the display surface. A large projection angle is desirable because itallows a more compact package, and it increases the range of angles overwhich the display can be viewed without projector components obstructingthe field of vision. As described in more detail below, folding opticsmay be used such that the projector may be located below the plane ofthe reflective screen, resulting in maximum compactness. These foldingoptics are omitted for clarity from FIG. 1A.

As shown in FIGS. 1B and 1C, angled projection introduces image defectsthat sharply increase with projection angle: such defects includekeystone distortion (see FIG. 1B) and anamorphic distortion (see FIG.1C). Not illustrated in FIGS. 1A-1C, but still significant, are visibleblur (when the dimension of the angled screen exceeds the focus depth ofthe projector), fall-off of light intensity with projection distance (sothat the luminance of the projected image is non-uniform), and otherwell-known optical aberrations such as chromatic aberration andastigmatism.

In the projection display of the present invention, some of theseartifacts may be corrected digitally. For example, keystone andanamorphic distortion, and fall-off of light intensity, may be correctedby projecting an image which has been pre-distorted, spatially and/or inbrightness, as is known in the art. Such digital correction comes at theexpense of some other attribute of the projected image. For example,spatial correction for keystone and/or anamorphic distortion will reducethe overall resolution of the projected image and light intensitycorrection will reduce image brightness. As discussed in more detailbelow, some artifacts (such as blur) are not amenable to digitalcorrection and must be corrected by a suitable choice of opticalelements.

The screens of most reflective displays, such as an electronic bookreaders, (E-readers) are rectangular. For reading a book, the screen isnormally used in portrait orientation (that is, with the longerdimension oriented towards and away from the user and the shorterdimension horizontally). Most commercially-available pico-projectionengines are also designed to project a rectangular image. However whensuch engines are projecting at an angle, the most compact packagedimensions (i.e., the smallest “throw ratio”, which is the distancebetween the projector and the screen divided by the diagonal dimensionof the screen) are achieved when the projector's landscape orientationis projected onto the portrait orientation of the reflective display, asshown in FIGS. 2A-2C.

FIG. 2A shows a light source 20 modulated by a light modulator 22 toform an image that is projected onto a screen 24. The modulator is inlandscape orientation, but its image is projected onto the screen whichis in portrait orientation.

FIG. 2B shows a light source 20 modulated by a light modulator 22 toform an image that is projected onto a screen 28. The modulator and thescreen are both in landscape orientation.

FIG. 2C shows a light source 20 modulated by a light modulator 26 toform an image that is projected onto a screen 24. The modulator is inportrait orientation and its image is projected onto the screen which isin landscape orientation.

It will be clear from FIGS. 2A-2C that the throw ratio is smallest, andtherefore the compactness of the device incorporating the screen and theprojector the greatest, when the long axis of the modulator is projectedonto the short axis of the screen, as shown in FIG. 2A. It is thuspreferred, in the projection display of the present invention, that theprojected and reflective images are in overall registration and haveapproximately the same width and height, the reflective display havingw1 pixels in the width dimension and h1 pixels in the height dimension,where

h1>w1

the light modulator of the projector having w2 pixels in the widthdimension and h2 pixels in the height dimension, where

h2<w2.

A disadvantage of the arrangement of FIG. 2A is that square pixels inthe light modulator 22 become anisotropic in the image projected ontothe screen 24, leading to a mismatch between the sizes of the projectedimage pixels and the (square) pixels on the reflective display screen24. For example, if the reflective screen is 600×800 pixels (SVGA), andeach pixel is square with a width of about 150 μm, while the lightmodulator of the projection engine is 848×480 pixels (WVGA), theresolution of the projected image cannot be more than about one halfthat of the image on the reflective display (i.e., the projected pixels,after correction for distortions as mentioned above, are on the order of300 μm in size, and are not square).

In a projection display of the present invention, the requirement thatpixels of the projected image be mapped 1:1 onto the pixels of thereflective display may be relaxed if the achromatic (luminance)information of the combined image is carried by only one of the hybriddisplay components, either the projected image or the reflective image,or if the spatial frequency content of one of the achromatic imagecomponents is reduced so that the effect of misalignment is no longervisible. This reduction in visibility of the mismatch in resolution ismade possible by the human visual system's different sensitivities ofachromatic (luminance) and chromatic image components to position andmotion. The chromatic acuity of the human visual system is significantlylower than its luminance acuity such that the perception of sharpness,fine detail and readability of text in displayed images is dominated bythe achromatic image component.

In a preferred embodiment of the projection display of the presentinvention, the achromatic image component is displayed on the reflectivedisplay because this display maintains its contrast under a wide rangeof ambient light levels, and the perceived contrast is even improved atvery high ambient light levels, such as sunlight. In order to render theluminance and chrominance information correctly using the projectiondisplay, the input image is separated into achromatic (black-and-white)and chromatic (color-only) components. Examples of color image encodingsystems that perform such a separation into one luminance component andtwo chrominance components include, but are not limited to, YCbCr, YIQ,YCC, CIELab, and oRGB. The following methods may be used to displaythese achromatic and chromatic components using a projection display ofthe present invention.

The achromatic component may be displayed on the reflective image; thechromatic components being projected by a color projector onto thisimage. The eye of the observer will recombine (fuse) the displayedachromatic and chromatic image components into a full color image. Sincethe chromatic acuity of the human eye is significantly lower than itsachromatic acuity, the perception of sharpness, fine detail andreadability of text will be dominated by the achromatic component. Ifthe chromatic components projected onto the reflective image have lowerresolution and sharpness than the achromatic component displayed and/orchromatic and achromatic components are out of registration, this willnot disturb the perception of sharpness, fine detail and readability ofthe combined image. In addition, the lower human visual sensitivity tomotion of the chrominance components ensures that small variations overtime of the relative positions of chromatic and achromatic imagecomponents, caused for example by vibration if theprojector-to-reflective image alignment is not completely rigid, willnot significantly disturb the perception of detail in the combinedimage.

In addition to chromatic image information, the projector engine may besupplied with achromatic (luminance) image information that has beenspatially filtered to improve overall image quality withoutre-introducing the registration requirement. For example, the achromatic(luminance) channel may be low-pass filtered (blurred) so that theeffects of misalignment and motion (vibration) between projected imageand reflective image remain invisible, but the contrast of the combinedimage is increased as compared with an image where only the chrominancecomponents are projected onto the reflective image.

These methods may be applied if the reflective image is a color imagewhich carries both luminance and chrominance information, while theprojected image carries only chrominance information, or chrominance andluminance information modified as described above.

FIG. 3 shows an optical design for a projection display (generallydesignated 30) of the present invention. The projection display 30comprises a projector module 31 that itself comprises a light source anda spatial light modulator, as described above, as well as associatedoptical elements such as beam splitters needed to produce modulatedimages in the three primary colors (red, green and blue). This image isprojected on to the viewing surface 38 of a reflective display using thefollowing optical elements:

-   -   (a) A projection lens or lens combination 32. The lens plane of        projection lens 32 meets the planes of the light modulator (in        the projector module 31) and of the reflective display 38        (corrected for the folding of the optical path by mirrors 36 and        37) at a common line (i.e., the modulator, projection lens 32        and reflective display 38 are arranged to meet the Scheimpflug        condition);    -   (b) An aspheric achromat combination 33 designed to provide        additional focusing of the projected image and to minimize        chromatic aberration (this combination may alternatively be        combined with projection lens 32);    -   (c) A toroidal lens 34, which minimizes astigmatism caused by        cone mirror 37 (described below). The lens 34 may be eliminated        if the mirror 37 has more complex curvature than a simple conic;    -   (d) A non-rotationally symmetric element 34, which provides        variable focusing power with field location;    -   (e) A folding mirror 36, which can be a plane mirror        (preferably) or curved (for instance, conic); and    -   (f) A cone mirror 37, which steers the beam to appropriate        locations on the viewing surface 38 of the display.

The optical elements in FIG. 3 are shown in positions to project animage from projector 31 on to viewing surface 38. When not in use, theymay be folded down to conserve space or the entire assembly comprisingelements 31-37 may be detachable from the reflective display.Alternatively, folding mirror 36 may be arranged to be removable fromthe optical path, or replaced by another optical element, such that theprojector 31 can project an image on to a distant surface instead of onto the viewing surface 38.

The optical elements shown in FIG. 3 may be replaced by diffractive orholographic elements, or by elements using nano-optical phasediscontinuity technology.

The location of the projection engine may be at the top, bottom or sideof the viewing surface 38 as viewed in portrait mode by a reader. If theprojection engine is located at the bottom of the viewing surface 38,other elements of the display, such as a keyboard, may be located abovemirror 37 such that the mirror is not visible to the reader. Althoughthe viewing surface 38 is shown as planar in FIG. 3, this is notessential since flexible reflective displays are well known in the art,and curvature of the display surface may be used to simplify or improvethe optical design described above. Additional elements, such as lightbaffles, may be incorporated to reduce stray light reflected specularlyfrom any of the surfaces of the display.

The projection display of the present invention may also includeelements required to drive the projector means and the reflectivedisplay. It is not necessary that the two displays always be drivensynchronously. Thus, for example, it may be desired to switch thereflective display to its white state (or possibly a gray state),completely or in part of its area, and to project an image onto thiswhite region using the embedded projector. This is desirable, forexample, if video rate content is to be viewed. In the current state ofthe art, the rate of switching of certain reflective displaytechnologies is not as high as that of projection engines.

When video addressing it is not necessary that the frame rates of thereflective image and the projected image match. For instance, theprojection engine may run at 60 frames per second but the reflectivedisplay may be run at 15 frames per second with a subset of the framesto enhance the contrast of the video.

FIG. 4 is a block diagram of one possible controller architecture of aprojection display of the present invention. The information to bedisplayed is loaded on to the controller by an input/output unit 4 andstored temporarily in memory 44 which also holds the necessary soft- andfirmware. A user interface 41 allows the viewer to control all necessaryfunctions such as loading, saving, selecting, and deleting ofinformation, and to initiate and progress through the viewing of theinformation. An Image and Video Signal Processing unit 45 conditions thestored information for display, and executes image and video processingalgorithms including, but not limited to, luminance and colorcorrection, separation into components for reflective and projectiondisplay, image rendering to the display and projector resolution,dithering, geometric pre-distortion, and uniformity correction. Theprocessed image signals are then transmitted to the necessary hardwarecontrollers. An electrophoretic display controller 48 controls thepixels of the reflective display 51; a micro-display controller 47controls those of the projector's image modulator 50. A Light EngineController 46 operates light sources 49 so that they can be adapted tothe projected color image or video, and to ambient illumination levels,and switched off when not needed to conserve power from battery 42 whichis supplied via a power controller 43.

The information display of the present invention will now be describedin more detail. This display may function as an outdoor informationdisplay suitable for all lighting conditions, across a wide temperaturerange, configurable to operate with no requirement for a mains powersource. The information display may consist of an electrophoreticdisplay for daylight conditions, complemented by LED or similar lightemitters for use in low light or other conditions. The light emittersmay also be required to assist in dim daylight conditions, or may beused at all times to provide desired colors. The light emitters may beintegrated into a front bezel or front illumination cavity to best suitthe environment.

The information display of the present invention desirably requires verylow power for operation, and the display may incorporate solar chargingelements for self-sustaining operation without an external power supply.Since some types of electro-optic material do not function well in lowtemperatures, it may be necessary to incorporate a layer of fronttransparent or rear thermal material into the electro-optic portion ofthe display. Alternatively the light emitters may be able to generatesufficient heat to keep the electro-optic material within its operatingrange. Other forms of heating of the electro-optic material may beactivated as needed to ensure proper switching.

The information display of the present invention is ideal to replaceexisting street signage, such as traffic lights and crosswalk signs,because of the benefits of being lighter weight, lower power, morevisible in solar glare, better vandalism resistance, ability to bedeployed easily in emergency environments, at a similar bill of materialcost to the prior art signage.

A simple traffic light system could be formed by three separateelectro-optic segmented cells each behind a complementary color filter.Such a system may include multiple light emitters for each cell, theselight emitters being directed towards the electro-optic displays in apattern to maximize the color visibility in low light conditions.

A simple cross walk sign could be of similar design to the trafficlight, except instead of a single segmented electro-optic cell, thesegmented cell could include multi-segmented icons or information.

These information displays can be made compatible with processor anddetection systems to synchronize the appropriate display informationwith the situational need. Control system options can be provided tomanage information wirelessly using low power and can incorporate solarcharging elements for self-sustaining operation without a mains powersupply.

The information display of the present invention offers the followingbenefits in comparison with prior art information displays:

-   -   reduction of system weight;    -   increased efficiency of power usage;    -   improved visibility in solar glare;    -   increased resistance to vandalism;    -   simpler deployment in emergency situations;    -   multiple improvements at a competitive bill of materials cost;        and    -   ability to scale in size with minimal design trade-offs.

FIG. 5 of the accompanying drawing is a side elevation of one cell(generally designated 100) of a three cell information display of thepresent invention in the form of a traffic light; part of the drawing isshown broken away to show internal details of the cell. The cellcomprises a substantially hemicylindrical visor 102 (best seen in FIG.6) which has a form similar to that in prior art traffic lights and alens 104. A circular monochrome electrophoretic display 106 is disposedat the rear surface of the cell 100 and is provided with a singleelectrode (not shown) on each of its major surfaces to enable thedisplay to function as a single pixel display. A plurality of lightemitting diodes (LED's) 108 are disposed at uniform intervals around theinternal surface of a collar 110 which surrounds the display 106 so thatthe light from the LED's 108 is directed on to the surface of thedisplay 106. The collar 110 and the LED's 108 are shown on a largerscale in FIG. 7. The LED's 108 are of a single color, red, amber orgreen depending upon the particular cell of the traffic light.

In normal operation, the LED's are driven continuously, and the phasesof the traffic light are controlled by switching the displays 106between their light and dark states.

The electrophoretic displays of the present invention will now bedescribed in more detail. As already mentioned, the third aspect of thepresent invention provides An electrophoretic display comprising: atleast one front electrode through which an observer can view thedisplay; a layer of an electrophoretic medium comprising a fluid and twotypes of electrically charged particles disposed in the fluid, one ofthe two types of particles being dark in color and the other beingreflective and of a color different from that of the dark particles; atleast one rear electrode disposed on the opposed side of the layer ofelectrophoretic medium from the front electrode(s), the rearelectrode(s) having a plurality of apertures extending therethrough; anda light source disposed on the opposed side of the rear electrodes fromthe layer of electrophoretic medium and arranged to direct light throughthe layer of electrophoretic medium. This display has a first opticalstate in which the dark particles lie adjacent the front electrode(s),so that the observer sees a dark color, a second optical state in whichthe reflective particles lie adjacent the front electrode(s), so thatthe observer sees the color of the reflective particles, and a thirdoptical state is which the dark particles lie adjacent the rearelectrode(s), the reflective particles the light source generates light,and the color of the reflective particles is visible to an observer.

This aspect of the present invention will be described below primarilyin its application as a brake light on a vehicle. However, theelectrophoretic display of the present invention is not limited to thisapplication, and may be used as any form of vehicle or traffic signage,or in other applications, such as warning lights on control panels.Basically, this electrophoretic display is designed to have the normaldark and colored states of a conventional dual particle electrophoreticdisplay, together with an additional emissive state (especially usefulin low light conditions) in which light from the light source passesthrough the electrophoretic medium and emerges displaying the color ofthe colored particles.

One electrophoretic display of the present invention (generally designed200) is illustrated in FIGS. 8 and 9 of the accompanying drawings. Thedisplay 200 has the form of a brake light for a vehicle and comprises amoisture barrier film 202 (which serves to protect the remainingcomponents of the display 200 from ambient moisture, including spraywhen the vehicle is driven in wet conditions), a substantiallytransparent front electrode 204, an adhesive layer 206 and a layer ofelectrophoretic medium (generally designated 208). The layer ofelectrophoretic medium 208 is an encapsulated electrophoretic mediumcomprising numerous capsules having capsule walls 210 enclosing adielectric fluid 212 in which are dispersed red particles 214 and blackparticles 216, the red and black particles bearing charges of oppositepolarity. Behind the layer 208 (i.e., to the left as illustrated in FIG.8 is a rear grid electrode 218; as best seen in FIG. 9, this gridelectrode 218 comprises wires 220 arranged in a hexagonal patternleaving most of the area of the electrode 218 occupied by hexagonalapertures 222. Finally, on the opposed side of the electrode 218 fromthe medium 208 is disposed a light source in the form of an incandescentbulb 224 equipped with a parabolic reflector 226. Although not shown inFIG. 8, the electrophoretic display comprising components 202-218 may besecured to the reflector 226 with a layer of optically clear adhesive;the bulb 224 and reflector 226 may form part of a prior art vehiclebrake light.

The display 200 is provided with a voltage source (not shown) forestablishing a potential difference between the electrodes 204 and 218.When it is not desired to display the brake light, the potentialdifference between the electrodes 204 and 218 is set to attract theblack particles 216 adjacent the electrode 204 and the red particles 214adjacent the electrode 218, so that the display assumes a first opticalstate in which the surface of the brake light appears dark. Note that inthis state it does not matter whether the bulb 224 is or is not lit,since no light will emerge from the display 200; however, to conservepower and increase bulb life, the bulb 224 will normally be turned off.

When it is necessary to turn the brake light on, the potentialdifference between the electrodes 204 and 218 is reversed, so that thered particles 214 lie adjacent the electrode 204 and the black particles216 adjacent the electrode 218. Thus, the display assumes a secondoptical state in which the red particles 214 reflect light incident onthe display and the brake light appears red and “lit”.

The explanation of the operation of the display 200 has so far assumedhigh ambient lighting conditions. In low light conditions, to turn thebrake light on, the potential difference between the electrodes 204 and218 is arranged so that the red particles 214 lie adjacent the electrode204 and the black particles 216 adjacent the electrode 218, and the bulb224 is illuminated so that the display assumes a third optical state inwhich light from the bulb 224 formed into a narrow beam by the parabolicreflector 226 passes through the red particles 214 adjacent theelectrode 204, thus causing red light to be emitted from the display andthe brake light to appear lit. Thus, the display 200 can achievesignificantly improved contrast in both low and high light conditions.

An appropriate drive scheme involving voltage or pulse width modulationmay be used in the display 200 to produce a state of defined visibility.Such a driving scheme could be synchronized to a clock or to a lightsensor or a temperature sensor to produce the desired visibility levelat any time of the day.

To provide an experimental test of an electrophoretic display of thepresent invention, a red and black pigment dispersion was preparedcontaining Solsperse 17k as a charging agent. The red pigment PaliotanRed L 3745, was treated with the silane Z6030 and coated withpoly(lauryl methacrylate) substantially as described in Example 28 ofU.S. Pat. No. 8,822,782. An electrophoretic medium comprising of 50weight percent pigment with 10:1 Red/Black ratio and 25 mg/gm ofSolsperse 17k in Isopar E was made and tested in a liquid test cell. Asshown in FIG. 10, the medium achieved a red state of 45a* and a darkstate of 10 L* (0.01% reflectance).

After replacing the backplane of the test cell with a transparent gridelectrode, the black pigment was observed to shutter in response to anapplied electric field. A movie of the transmission through the testcell was acquired on a camera and variable transmission through thedevice was clearly visible.

The electrophoretic medium of the present invention is not, of course,confined to the use of red particles. Provided one of the particles ishighly absorbing, the other particle may be reflective (white), colored,retro-reflective or transparent. One or more dyes may also be includedin the fluid to achieve desired color states in the display.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. An electrophoretic display comprising: a front electrode throughwhich an observer can view the display; a layer of an electrophoreticmedium, the electrophoretic medium comprising a fluid and two types ofelectrically charged particles disposed in the fluid and capable ofmoving through the fluid on application of an electric field to thefluid, one of the two types of particles being dark in color and havinga first charge, and the other being reflective and of a color differentfrom that of the dark particles and having a second charge that isopposite the first charge; at least one rear electrode disposed on theopposed side of the layer of electrophoretic medium from the frontelectrode, the rear electrode having a plurality of apertures extendingtherethrough; and a light source disposed on the opposed side of therear electrodes from the layer of electrophoretic medium and arranged todirect light through the layer of electrophoretic medium, the displayhaving a first optical state in which the dark particles lie adjacentthe front electrode, so that the observer sees a dark color, a secondoptical state in which the reflective particles lie adjacent the frontelectrode, so that the observer sees the color of the reflectiveparticles, as illuminated by ambient light, and a third optical state inwhich the dark particles lie adjacent the rear electrode, the lightsource generates light, and the color of the reflective particles,illuminated by the light source, is visible to an observer.
 2. Theelectrophoretic display of claim 1, wherein the rear electrode comprisesa grid electrode.
 3. The electrophoretic display of claim 1, wherein thedark particles are black and the reflective particles are red.
 4. Avehicle including an electrophoretic display according to claim 1.